1. Field of the Invention
The present invention relates to zoom lenses and optical apparatuses, such as cameras, which use the zoom lenses. More specifically, the present invention relates to a zoom lens suitable for use in optical apparatuses such as video cameras, electronic cameras including digital still cameras, and film cameras.
2. Description of the Related Art
Recently, in accordance with the increase in functionality of optical apparatuses such as video cameras using solid-state imaging devices and electronic still cameras, small, high-resolution zoom lenses are required as imaging systems for such optical apparatuses. In order to satisfy these requirements, small zoom lenses having a relatively high zooming ratio are disclosed in, for example, Japanese Patent Laid-Open Nos. 56-114920 (U.S. Pat. No. 4,256,381) and 58-160913 (U.S. Pat. No. 4,720,180). These zoom lenses include four lens units, i.e., a first lens unit having positive refractive power, a second lens unit having negative refractive power, a third lens unit having positive refractive power, and a fourth lens unit having positive refractive power, which are arranged in that order from the object side to the image side. The first, second and fourth lens units are moved for zooming. With such an arrangement, a relatively high zooming ratio can be obtained while the size of the zoom lens is reduced.
In addition, Japanese Patent Laid-Open Nos. 62-24213 and 62-247316 also disclose zoom lenses including four lens units, i.e., a first lens unit having positive refractive power, a second lens unit having negative refractive power, a third lens unit having positive refractive power, and a fourth lens unit having positive refractive power, which are arranged in that order from the object side to the image side. In these zoom lenses, the second lens unit is moved for zooming and the fourth lens unit is moved to compensate for displacement of the image plane due to zooming. The fourth lens unit is also used for focusing.
Japanese Patent Laid-Open Nos. 4-14006 (U.S. Pat. No. 5,134,524) and 4-358108 (U.S. Pat. No. 5,341,243) disclose zoom lenses including a first lens unit having positive refractive power, a second lens unit having negative refractive power, an aperture stop, a third lens unit having positive refractive power, and a fourth lens unit having positive refractive power, which are arranged in that order from the object side to the image side. In these zoom lenses, the first lens unit is moved toward the object side and the second lens unit is moved toward the image side for zooming from a wide-angle end to a telephoto end, and the fourth lens unit is moved for zooming and focusing. The aperture stop is always fixed during zooming.
Japanese Patent Laid-Open No. 11-242160 (U.S. Pat. No. 6,101,043) also discloses a zoom lens including four lens units, i.e., a first lens unit having positive refractive power, a second lens unit having negative refractive power, a third lens unit having positive refractive power, and a fourth lens unit having positive refractive power, which are arranged in that order from the object side to the image side. In this zoom lens, the first and fourth lens units are moved toward the object side and the second lens unit is moved toward the image side for zooming from a wide-angle end to a telephoto end. The third lens unit and the aperture stop are always fixed during zooming.
Japanese Patent Laid-Open No. 4-14007 (U.S. Pat. No. 5,134,524) also discloses a zoom lens including a first lens unit having positive refractive power, a second lens unit having negative refractive power, an aperture stop, a third lens unit having positive refractive power, and a fourth lens unit having positive refractive power, which are arranged in that order from the object side to the image side. In this zoom lens, the first lens unit is moved toward the object side and the second lens unit is moved toward the image side for zooming from a wide-angle end to a telephoto end, and the fourth lens unit is moved for zooming and focusing. The aperture stop is moved toward the image side for zooming from an intermediate focal length to the telephoto end.
U.S. Reissue Pat. No. 32,923 also discloses a zoom lens including a first lens unit having positive refractive power, a second lens unit having negative refractive power, an aperture stop, a third lens unit having positive refractive power, and a fourth lens unit having positive refractive power, which are arranged in that order from the object side to the image side. In this zoom lens, the first and fourth lens units are moved in the same direction for zooming, and the aperture stop is fixed during zooming.
In general, when the refractive power of each lens unit included in a zoom lens is increased, the amount by which each lens unit must be moved in order to obtain a predetermined zooming ratio is reduced. Accordingly, the size of the zoom lens can be reduced and the zooming ratio thereof can be increased at the same time.
However, when the refractive power of each lens unit is increased in this simple manner, a problem occurs in that variations in aberrations which occur during zooming are increased, and it becomes difficult to obtain high optical performance over the entire zooming range, especially when a high zooming ratio is required.
On the other hand, when rear focusing, in which lens units other than the first lens unit are used for focusing, is employed in a zoom lens, the overall size of the lens system advantageously can be reduced, focusing can be performed at high speed, etc.
However, it becomes difficult to obtain a high zooming ratio, and variations in aberrations which occur during zooming are increased. Accordingly it becomes extremely difficult to obtain high optical performance over the entire range of the distance to the object, that is, from an infinitely distant position to a close position.
Especially when a high zooming ratio is required, it becomes difficult to obtain high optical performance over the entire zooming range, and over the entire range of the distance to the object.
In order to overcome the above-described problems of zoom lenses of the known art, an object of the present invention is to provide a zoom lens in which the overall size of the lens system is reduced, which has a high zooming ratio, and which exhibits high optical performance.
In order to attain the above-described objects, a zoom lens of the present invention includes, in order from the object side to the image side: a first lens unit having positive optical power; a second lens unit having negative optical power; a third lens unit which has positive optical power and which does not move for zooming; and a fourth lens unit having positive optical power. In zooming from a wide-angle end (e.g., where the zoom lens is arranged to perform a maximum wide-angle adjustment) to a telephoto end (e.g., where the zoom lens is arranged to perform a maximum telephoto adjustment), the first lens unit is moved toward the object side and the second lens unit is moved toward the image side, whereby the distance between the first and second lens units is increased. In addition, when fW and fT are focal lengths of the overall system at the wide-angle end and the telephoto end, respectively, f2 is a focal length of the second lens unit, e3T is the distance between principal points of the third and fourth lens units at the telephoto end when an object located at infinity is in focus, f3 and f4 are focal lengths of the third and fourth lens units, respectively, and fA is defined as fA={square root over (fwxc2x7fT)}, at least one of the following expressions is satisfied:
xe2x80x830.17 less than |f2/fA| less than 0.30,
2.05 less than e3T/fW less than 2.96, 
and
0.6 less than f3/f4 less than 0.92. 
Further objects, features and advantages of the present invention will become apparent from the following description of the preferred embodiments with reference to the attached drawings.